Free machining stainless steel alloy

ABSTRACT

Stainless steel alloy containing about 10 to 30 percent chromium, 0.015 to 0.75 percent sulfur and/or selenium, 0.5 to 7 percent copper, 0.25 to 4 percent aluminum, 0.001 to 0.75 percent tellurium and the balance iron and optional elements. Ferritic and martensitic compositions are provided having improved free machining properties or an improved combination of free machining and corrosion-resistance properties in selected media.

United States Patent Aulenbach et a1.

[451 Feb. 29, 1972 [54] FREE MACHINING STAINLESS STEEL ALLOY [72] Inventors: Grant M. Aulenbach, Mohnton; Kermit J.

Goda, Jr., Leesport, both of Pa.

Carpenter Technology Corporation, Reading, Pa.

[22] Filedi Sept. 4, 1969 [21] Appl.No.: 855,416

[73] Assignee:

[52] U.S. Cl ..75/l24, 75/125, 75/126 M, 75/128 P [51] lint. Cl ..C22c 39/20, C22c 39/14 [58] FieldoiSearch ..75/126 L, 126 M, 126 P, 123 AA, 75/125, 128 P [.561 sfe ve Cited *i'J'N'iTED STATES PATENTS 3,437,478 4/1969 Moskowitz ..75/128 R 2,009,713 7/1935 Palmer 1,846,140 2/1932 Palmer ..75/l28 P Primary Examinerl-lyland Bizot Attorney-Edgar N. Jay

[57] ABSTRACT FREE MACHINING STAINLESS STEEL ALLOY This invention relates to stainless steel alloys and more particularly to ferritic chromium-bearing steel alloys and martensitic chromium-bearing steel alloys having improved free machinability or improved combined properties of free machinability and corrosion resistance in selected media.

Chromium-bearing stainless steel containing little or no nickel may contain from about to 30 percent chromium. Depending upon the amount of chromium and the other alloying elements present, the microstructure of the alloy may be ferritic or martensitic, or both phases can be present in varying proportions. Because of their mechanical and chemical properties, and in some instances their magnetic properties, such alloys are highly desired for a wide variety of uses. Because of the difficulty encoimtered in machining such alloys, standard grades have been modified by means of freemachining additives for use where the accompanying reduction in corrosion resistance could be tolerated. For example, both A.l.S.I. type 416 and type 430F are illustrative of grades which contain substantial amounts of sulfur or selenium, that is more than about 0.15 percent and usually 0.3 percent or more, for improved machinability.

A number of other elements have also been used in combination with or in place of the elements sulfur and selenium, but the results left much to be desired usually because the increase in the cost of the alloy and/or the detrimental effect on other properties such as corrosion resistance were not sufficiently offset by the gain in free machining properties. For example, U.S. Pat. No. 2,897,078 is illustrative of chromiumbearing stainless steels in which the elements phosphorus, sulfur and selenium are considered equivalents, in which the permissible amount of manganese is increased to 2.0% and to which is added 0.5-3.0% copper, 0.5-3.0% aluminum and 0.05-l.O% zirconium and/or molybdenum. As stated in that patent, manganese is used because of its beneficial effect on the free-machining properties of the alloy in spite of its detrimental effect on corrosion resistance. We have also noted that manganese, probably because of the relative solubility of manganese sulfides, has an adverse effect on corrosion resistance, but we found that it could be offset to some extent by carefully passivating the surface of the part. As would be expected, as the passivated surface from which the manganese sulfides has been removed wears away, the underlying material becomes exposed and is attacked.

Our present invention stems from our discovery that when a relatively small amount of tellurium is present together with the elements copper and aluminum, they work together with sulfur or selenium or both of them to impart a unique degree of free machinability to chromium-bearing stainless steel alloys without reducing the corrosion resistance to the extent expected from previous experience with the sulfur additions required to attain a like degree of machinability.

It is therefore a principal object of the present invention to provide ferritic and martensitic stainless steels capable of providing a degree of free machinability at least about equal to that of currently available corresponding standard grades of free machining alloys but with improved corrosion resistance.

It is a further object of this invention to provide such stainless steel alloys having a degree of corrosion resistance at least about equal to that of corresponding grades of free machining stainless steels, but having substantially improved free machinability.

The foregoing as well as additional objects and advantages of our invention are achieved by providing a chromium stainless steel alloy containing the following elements in the amounts indicated in the broad and preferred ranges given in approximate weight percent in the following table:

Copper 0.5-7 0.75-4 Aluminum 0.254 0.50-L25 Tellurium 0.00|-0.75 0.01-0.]

In addition, there can be included varying amounts of other elements, as for example: up to about 3% silicon but preferably no more than about 1%, up to about 0.5% preferably no more than 0.035% phosphorus, up to about 2.5% nickel, up to about 5% preferably no more than l.5% molybdenum, or tungsten can replace all or part of the molybdenum in the ratio of about 2 to 1, up to about 0.01% boron, and up to about 1% preferably no more than 0.5% each of columbium or titanium. Except for incidental impurities, the balance of the alloy is iron.

The elements sulfur and/or selenium together with controlled amounts of copper, aluminum and tellurium work together in our composition to provide an unexpected degree of free machinability and corrosion resistance. While sulfur in amounts ranging from about 0.015 to 0.75 percent can be present, above about 0.5 percent increasing difficulty may be encountered in both hot and cold working the composition. We therefore preferably limit sulfur to no more than about 0.4 percent to minimize working difficulties. Selenium on a l-for- 1 basis can be substituted for all or part of the sulfur in our composition as is indicated in the foregoing tabulation where the ranges stated are to be read as the broad and preferred amounts of the combined content of both sulfur and selenium. The elements sulfur and selenium, individually or together, are not equivalent to and cannot be substituted for the element tellurium in our composition.

Copper can be present in solid solution in our composition or out of solution as a copper-rich phase or both forms can be present. Its most beneficial effect on free machinability seems to occur when at least some of the copper is present out of solution and forms a copper-rich phase which acts as a chip breaker. Below about 0.5 percent, there is not enough copper present to provide a significant effect while above about 7 percent, increasingly severe segregation problems appear which cause hot shortness.

Aluminum contributes to the formation of smaller and thereby better dispersed sulfides. Because aluminum is a powerful ferrite former, its use must be carefully controlled in compositions which are intended to be entirely or primarily martensitic. Broadly, from about 0.25 to 4 percent aluminum can be present in our composition. Larger amounts of aluminum cause embrittlement and excessive grain size. In order to facilitate maintaining the martensitic balance in some compositions, less than 2 percent aluminum should be used. We prefer to use aluminum in an amount ranging from about 0.50 to 1.25 percent.

Our experiments show that in the absence of at least a small but effective amount of tellurium, the outstanding free machinability of our composition cannot be attained. We preferably use about 0.01 to 0. l% tellurium although as little as 0.005% or even 0.001% and as much as 0.75% tellurium can be used. To obtain the beneficial effect of tellurium, it is necessary that the elements copper and aluminum also be present within the limits stated. The tellurium appears to form tellurium-rich compounds, which may be tellurides, attached to the sulfides. The more tellurium present, the more the sulfides are surrounded by the tellurium-rich compounds.

By the addition of the elements copper, aluminum, and tellurium in accordance with out invention, the sulfur content of a given alloy can be reduced so as to obtain better corrosion resistance with little or no loss in free machinability or even in some instances with some improvement in machinability. On the other hand, when the previously acceptable sulfur level is maintained, the addition of the elements copper, aluminum and tellurium makes possible significantly improved free machinability.

Because of its adverse effect on corrosion resistance, manganese is preferably not added to our composition. If added, manganese is kept below about 5%. When its beneficial effect on free machinability is wanted, manganese can be added where the resultant impairment of corrosion resistance can be tolerated, and for this purpose is included in the range of about 0.4 to 2.5 percent. The effect of manganese on corrobility, as measured by the drill penetration test was 415 as compared to the value of 45 1' obtained with specimens of Example l0. The small differences noted in the hardnesses of Example l-l 1 are not considered significant.

sion resistance seems to be mainly that of increasing the solu- 5 Modification of A.l.S.I. type 416 in accordance with the bility of the sulfides in dilute acids. Having in mind that free present invention is defined by the following intermediate machining stainless steel grades are not intended for use range: where, as in chemical-processing equipment, they would be exposed to very rigorous media, the smaller amounts of man- Weight Percent ganese, from about 0.4 percent to about 0.6 percent to 0.7 gr M015 2 M I v romlum percent, provide better corrosion resistance than when man- Sum" pmsselcnium 0154's ganese is present in the larger amounts. Copper 0,754

The examples having the analyses in approximate weight Aluminum 0.5-1.25 percent as shown in Table I serve to illustrate our invention. Tellmum Manganese up to abo ut 2.5 TABLE I Drill Hard- Si P S Cr Ni M0 011 Al Te test I mass 1 111 thoussndths of an inch. 2 Rockwell B Scale. I

. S'l' u t b utl Examples 1-9 and 11, except for the amounts of the ele- 23: 5,: 1,3,, ments copper, aluminum and tellurium present, will be recog- Nickel up to about 0.5 nized as falling within the limits of A.I.S.l. type 416, a stainless 4 Columbium up to about 0.25 steel alloy known for its free machinability and which is illus- .riwnium up w about 015 trated by Example 10. Similarly, Example 15 illustrates Molybdenum ammo,

A.l.S.l. type 430F and Examples 12-14 differ therefrom by the addition of copper, aluminum and tellurium in accordance with the present invention.

ingots were cast of each of the foregoing examples, hot worked and shaped to form test pieces which were annealed before testing. Examples l] l were annealed in the temperature range of 1,300-l,450 F. for 1 hour per inch of material followed by air cooling. Examples l2-l5 were annealed in the temperature range of 1,400l,500 F. for 1 hour per inch of material and were also air cooled. The hardness of the examples in their annealed condition was measured on the Rockwell B Scale, and the results are recorded in the righthand column.

The machinability of the specimens of each of the examples was determined as the average depth of penetration in thousandths of an inch into the specimens under carefully controlled conditions. While there is Test, accepted standard for measuring machinability, the free machining values were obtained by measuring the depth of penetration into the specimens by a quarter-inch drill in a time intrval of 15 seconds with the drill rotating at or very close to 670 r.p.m. under constant torque. Before the start of each drilling operation, the drill mounted in a conventional drill press was brought against the surface of the specimen where it was maintained by a constant weight of 100 pounds. The results of the, tests are recorded in Table l under Drill Test", and each is an average of three tests.

On comparison with Example 10, Examples l-9 clearly demonstrate the improvement in machinability obtained in accordance with the present invention when, with the sulfur content held at about the same level or somewhat reduced, the elements copper, aluminum and tellurium are added. The improvement in free machinability is attained without any significant effect on the corrosion resistance of the composition as compared to that of A.l.S.l. type 4l6. The elements copper, aluminum and tellurium acting alone or only two at a time cannot provide this effect. For example, with about 2 percent copper and about 1 percent aluminum in Example ll, comparable in all other respects with Example lt), f 'r e e machi na and the balance iron except for incidental impurities or other additions which do not adversely affect the desired properties of the composition.

Examples 12-15 demonstrate the improved free machining properties obtained by modifying A.l.S.l. type 430F alloy in accordance with the present invention, which properties are accompanied by an improvement in the corrosion resistance properties of .the composition. These improved properties are provided in accordance with the present invention by the addition of copper, aluminum and tellurium in the amounts indicated for better free machinability and by a reduction in the sulfur content by more than about 50 percent to obtain better corrosion resistance. It is to be noted that the hardness of Example 15 is significantly lower than that of Examples 12-14, and, having in mind that machinability as measured by the drill test decreases as the hardness increases from R 81, the improvement in free machinability is greater than appears from a direct comparison of the drill test results of Examples l2-l5. Furthermore, Example 12 with a value of 0.358 in. average from the drill test at a hardness of R 88 actually demonstrates improved free machinability as compared to Example 15 when the difference in hardness is taken into account. The following intermediate range defines the modification 'of A.I.S.l. type 430F in accordance with the present invention:

Weight Percent Carbon up to about 0.l2

Chromium l4-l8 Sulfur plus Selenium (HS-0.5 pp 1 1.. Aluminum 0.5-! .25

Tellurium 0.0l0.l

Manganese up to about 2.5

Silicon up to about I Phosphorus up to about 0.2

Nickel up to about 0.5

was. 9.9 WWI-9L Columbium up to about I Titanium up to about I Molybdenum up to about 0.6

and the balance iron except for incidental impurities or other additions which do not adversely affect the desired properties of the composition.

When tested in a 10 percent by weight nitric acid dip at room temperature after passivating in a solution of 20 percent by weight nitric acid and 2% by volume Na Cr O at 120 F. for one-half hour, none of the specimens of Examples 12-15 became discolored. When tested in a percent by weight salt spray at 95 F. after the same passivating treatment, two

specimens of Example 15 rusted after an average of about 2 hours, two specimens of Example 12 rusted after an average of about 4 hours, four specimens of Example. 13 gave an average of about 38 hours with one of the specimens showing no rust after 50 hours exposure when it was removed from the test medium, and of the two specimens of Example 14 tested, one rusted after 48 hours, and the other was removed from the test medium after 50 hours exposure without rusting. When prior to testing in the 5 percent by weight salt spray, passivated specimens of each of the examples were first subjected to a by weight NaOl-l dip at 120 F. for 1 hour, the specimens of Example 15 yielded an average of about 21 hours, Example 12 yielded an average of about 35 hours with one specimen being removed from the test medium after 50 hours without rusting, Example 13 yielded an average of about 37 hours with two of the four specimens tested being removed from the test medium after 50 hours without rusting, and in the case of Example l4, both specimens tested were removed from the test medium after 50 hours without rusting.

Thus, for good free machinability combined with better corrosion resistance than hitherto, we limit sulfur in our composition to less than 0.25 percent and preferably to no more than about 0.15 percent to 0.20 percent.

vln the case of wholly ferritic compositions of the present invention, balanced so as not to transform to austenite to any significant extent at high temperature, hot workingcan be carried out without any unusual difficulties by heating to the normal hot work range, that is the temperature range in which the composition is most ductile, and then forging in the customary manner. Examples 12-l4 are illustrative of such compositions.

0n the other hand, our compositions which are wholly ornormal hot working temperature, the microstructure is less than about 50 percent ferrite and more than 50 percent austenite. As is well known, the forging temperature for the martensitic chromium steels usually ranges from about 2,l00 F. to about 2,250 F. and, in some instances, closer to 2,300 F. depending upon the alloy content. However, the martensitic compositions of the present composition which contain less than about 50 percent ferrite at the normal hot working temperature tend to develop body and corner tears (similar to those found in an alloy that is hot short) presumably due to the presence of a low melting point constituent and require special treatment. in the case of those compositions, hot working does not get worse as the forging temperature is raised as in the case of the usual hot short compositions. On the contrary, those hot working difficulties disappear, and forging can be successfully carried out at temperatures above about 2,300 F. Generally stated, the hot working range should be above the delta ferrite transformation point (the start of the austenite to delta ferrite transformation). The preferred temperature is at about 2,400 F.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

We claim:

Stainless sisl y in see .trsenashiaebiliu in or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of 2/l, and the balance essentially tron.

2. A stainless steel alloy as set forth in claim 1 containing about 0.02-0.4% sulfur plus selenium, about 0.75-4% copper, about 0.50-1 25% aluminum, and about 0.010.l% tellurium.

' 3. A stainless steel alloy as set forth in claim 2 containing about:

. Weight Percent Carbon 0.08-1 Chromium l0-20 Manganese up to 2.5 Silicon up to l Columbium up to 05 Titanium up to 0.5 Molybdenum up to L5 4. A stainless steel alloy as set forth in claim 3 containing no more than about 0. 1 5-020 percent sulfur plus selenium.

5. A stainless steel alloy as set forth in claim 2 containing less than 0.25 percent sulfur plus selenium.

40 6. A stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of:

Carbon 0.08-1 5 Chromium l0-20 Sulfur plus Selenium 0.0l5-0.75 Copper 0.5-7 Aluminum 0.25-4 Tellurium 0.00l-0.l Manganese up to 2.5 Silicon up to l Phosphorus up to 0.035

Nickel up to 2.5 Boron up to 0.0l Columbium up to 0.5 Titanium up to 0.5 Molybdenum up to L5 and the balance essentially iron.

7. A stainless steel alloy as set forth in claim 6 containing about 0.4-0.6 to 0.7 percent manganese.

8. A stainless steel alloy as set forth in claim 7 containing less than 0.25 percent sulfur plus selenium.

9. A stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of:

Carbon up to about 0.15 Chromium l2-l4 Sulfur plus Selenium 0.l5-0.5 Copper 0.75-4 Aluminum 0.5-L25 Te u iu -Ol-DJ Manganese up to about 2.5 Silicon up to about I Phosphorus up to about 0.2 Nickel up to about 0.5 Boron up to about 0.01 Columbium up to about 0.25

Titanium up to about 0.25 Molybdenum up to about 0.0 I

and the balance essentially iron. at least about 0.005 percent tellurium.

10. A stainless steel alloy having good free machinability in 12. A stainless steel alloy as set forth in claim 11 containing its annealed condition consisting essentially in approximate about: weight percent of:

5 Carbon up to about 0.12 welsh Percent Chromium I448 Sulfur plus Selenium 0.lS-0.5 Carbon 93- ppe 0775-4 v Chromium l0-20 Aluminum 0.5l.25 Manganese up m 5 Tellurium 0.o|-0.| Silicon up w Manganese up to about 2.5 Columbium up m 05 Silicon up to about 1 Timnium up m 0.5 Phosphorus up to about 0.2 L5 Nickel up to about 0.5 Molybdenum up 0 Boron up to about 0.01 Columbium up to about I Titanium u to about I Molybdenum ammo, 13. A stainless steel alloy as set forth in claim 12 containing about 0.02-0.4% sulfur plus selenium, about 0.754% copper, and the balance essentially iron. and about 0.5-1 25% aluminum 11. A stainless steel alloy as set forth in claim l containing a =r (SEAL) Attest: v v

' EDWARD M.FLEJTDPJJR,JR. C. MARSHALL DANN Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated h Imam! 29 1 912 Inventor(s) Grant M. Aulenbach & Kermit J. Goda Jr.

Patent No. 3 45 322 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 65, for "out" read our Column 3, line 53, for "Test,"" read no generally Signed and sealed this 9th day of April 19714..

Attesting Officer USCOMM'DC 60376-P69 U.S. GOVERNMENT PRINTING OFFICE: I969 0-366-334 FORM PO-105O (10-69) 

2. A stainless steel alloy as set forth in claim 1 containing about 0.02-0.4% sulfur plus selenium, about 0.75-4% copper, about 0.50-1.25% aluminum, and about 0.01-0.1% tellurium.
 3. A stainless steel alloy as set forth in claim 2 containing about: Weight Percent Carbon 0.08-1 Chromium 10-20 Manganese up to 2.5 Silicon up to 1 Columbium up to 0.5 Titanium up to 0.5 Molybdenum up to 1.5
 4. A stainless steel alloy as set forth in claim 3 containing no more than about 0.15-0.20 percent sulfur plus selenium.
 5. A stainless steel alloy as set forth in claim 2 containing less than 0.25 percent sulfur plus selenium.
 6. A stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of: Carbon 0.08-1 Chromium 10-20 Sulfur plus Selenium 0.015-0.75 Copper 0.5-7 Aluminum 0.25-4 Tellurium 0.001-0.1 Manganese up to 2.5 Silicon up to 1 Phosphorus up to 0.035 Nickel up to 2.5 Boron up to 0.01 Columbium up to 0.5 Titanium up to 0.5 Molybdenum up to 1.5 and the balance essentially iron.
 7. A stainless steel alloy as set forth in claim 6 containing about 0.4-0.6 to 0.7 percent manganese.
 8. A stainless steel alloy as set forth in claim 7 containing less than 0.25 percent sulfur plus selenium.
 9. A stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of: Carbon up to about 0.15 Chromium 12-14 Sulfur plus Selenium 0.15-0.5 Copper 0.75-4 Aluminum 0.5-1.25 Tellurium 0.01-0.1 Manganese up to about 2.5 Silicon up to about 1 Phosphorus up to about 0.2 Nickel up to about 0.5 Boron up to about 0.01 Columbium up to about 0.25 Titanium up to about 0.25 Molybdenum up to about 0.6 and the balance essentially iron.
 10. A stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of: Carbon up to about 0.12 Chromium 14-18 Sulfur plus Selenium 0.15-0.5 Copper 0.75-4 Aluminum 0.5-1.25 Tellurium 0.01-0.1 Manganese up to about 2.5 Silicon up to about 1 Phosphorus up to about 0.2 Nickel up to about 0.5 Boron up to about 0.01 Columbium up to about 1 Titanium up to about 1 Molybdenum up to about 0.6 and the balance essentially iron.
 11. A stainless steel alloy as set forth in claim 1 containing at least about 0.005 percent tellurium.
 12. A stainless steel alloy as set forth in claim 11 containing about: Weight Percent Carbon 0.08-1 Chromium 10-20 Manganese up to 2.5 Silicon up to 1 Columbium up to 0.5 Titanium up to 0.5 Molybdenum up to 1.5
 13. A stainless steel alloy as set forth in claim 12 containing about 0.02-0.4% sulfur plus selenium, about 0.75-4% copper, and about 0.5-1.25% aluminum. 